Steam has a built in cheat detector that prevents you from using in game chat cheats to earn achievements in the campaigns, however it doesn't work properly!
You must complete all of the levels, and you can cheat using the chat and entering "i r winner".
Once you have beaten all the levels in this campaign, exit the entire game and reload the game.
You then need to complete one of the levels without cheating to confuse the game into thinking you've completed all the levels. You can choose any of the six.
The fastest level is almost certainly level 1. Any difficulty is acceptable.
A bit weird really, but it saves virtual bloodshed.
